Intent on undermining his Democratic rival, Trump and GOP backers are increasingly relying on rhetoric that academics and even some Republican strategists say has an undeniable edge focused on gender. His criticism of Clinton goes beyond "Crooked Hillary," and complaints about her use of a private email server as secretary of state and her foreign policy decisions.

Clinton, Trump said in a speech last week, "lacks the mental and physical stamina to take on ISIS and all the many adversaries we face."

He has repeatedly called attention to Clinton's voice, saying listening to her gives him a headache. Last December, he mocked her wardrobe. "She puts on her pantsuit in the morning," he told a Las Vegas audience. At rallies and in speeches, the billionaire mogul has also used stereotypes about women to demean Clinton, who stands to become America's first female president if she wins in November.

A frequent point of criticism: Clinton doesn't look like a typical president.

"Now you tell me she looks presidential, folks," he said at a recent rally in New Hampshire.

"I look presidential," he insisted.

Trump's allies have piled on. Running mate Mike Pence often uses the word "broad-shouldered" to describe Trump's leadership and foreign policy style, a tacit swipe at Clinton. Former New York Mayor Rudy Giuliani argued that all of the miles Clinton logged during as secretary of state resulted in more harm than benefit.

"Maybe it would've been better if she had stayed home," said Giuliani, who more recently questioned Clinton's health, suggesting an internet search of the words "Hillary Clinton illness."

"She is the first woman from a major party running for president, so gender is always at play," said Dianne Bystrom, the director of the Carrie Chapman Catt Center for Women and Politics at Iowa State University.

Clinton pushed back Monday against insinuations she's in poor health, saying on ABC's "Jimmy Kimmel Live" that campaign is like an "alternative reality" where she has to "answer questions about am I alive, how much longer will I be alive, and the like."

Gender has always been tricky for Clinton. Throughout her career, she has struggled with how to confront gender norms, ranging from the extent to which to embrace the historic potential of her candidacy to whether she should be referred to by her married name.

Trump, meanwhile, has accused Clinton of "using the woman card" to boost her appeal. That may not have helped his standing with female voters: A recent survey by the Pew Research Center found Clinton with a 19-percentage-point lead over Trump among women. Trump, meanwhile, has a 12-percentage-point advantage with men.

Katie Packer, a Republican strategist who founded a political consulting firmed aimed at appealing to Republican women, said that Trump has a history of seizing on his rivals' perceived weaknesses. In Clinton's case, she said, that appears to include her sex.

"He clearly views women as sort of the weaker sex, so I think he's going to look to exploit that with Hillary," said Packer, who helped run an independent anti-Trump organization during the primaries.

Trump's rallies are filled with blatant misogyny. Supporters wear "Trump vs. Tramp" political buttons, and have even harsher slogans and signs.

At the same time, Trump has a long history of hiring female executives and last week became the first Republican in the party's history to appoint a woman, pollster Kellyanne Conway, as his campaign manager.

Trump's campaign did not respond to requests for comment Monday, though in the past he has dismissed sexism charges as "nonsense."

Yet Conway herself has advocated using Clinton's sex against her.

Speaking to The New York Times in April, when she was still backing a Trump rival, Conway said Trump's efforts to turn Clinton's gender against her could prove effective.

"By taking gender head-on, Trump refuses to cede women voters and so-called women's issues to Hillary just because she is a woman," she told the paper. "He is 'Swiftboating' her by throwing shade on what should be a strength." Her mention of "Swiftboating" was referring to widely debunked criticism of Democratic nominee John Kerry's war record in the 2004 campaign.

Kelly Dittmar, a scholar at the Rutgers University's Eagleton Institute of Politics' Center for American Women and Politics, who has been tracking the gender dynamics in the race, said that even during the primaries when Trump was competing mostly against men, he took on the role of strong man, demeaning rivals.

"His message has been: I'm the manliest candidate, I'm the strongest, I know how to protect women — which is a pretty paternalistic take on it — I'm going to destroy ISIS and be very tough, to the point where he's talking about the size of his own manhood," she said of the candidate. "If you're trying to prove you're the manliest, then you're trying to emasculate your opponent."